I wonder if it's "Other Women" by Lisa Alther. I read it more than once, if I'm honest enough to admit this, many times, in my 30's. It was one of my comfort books that I read over and over, when I had insomnia or any other head whirly anxiety. It's also really well written. I like the other books I've read of hers, especially Kinflicks.

More info and it looks like you can get it used for $3.

Other Women: Lisa Alther: 9780452276789: Amazon.com: Books